The present invention relates generally to radio frequency (RF) transmitter systems for use in cellular telephone handsets and other wireless communications devices. More particularly, the present invention relates to circuits, employed to control the operation of power amplifiers within the transmitter systems.
In recent years, the usage of cellular telephones and other wireless communications devices has grown dramatically. Such wireless devices must be able to transmit radio frequency (RF) signals of sufficient strength that the signals can be received at great distances from the wireless devices. To generate such high-intensity output signals, the wireless devices typically include amplifier circuits that amplify low-intensity input signals which have been processed within the wireless devices. To effectively amplify the input signals, the amplifier circuits must amplify the input signals in a linear manner, to avoid distortion.
Due to the limitations inherent in conventional batteries, wireless devices have limited access to power. Consequently, power efficiency is critical to improving the performance of wireless devices, particularly in terms of the timespan over which the wireless devices can operate without being recharged. The amplifier circuits used to generate the high-intensity output signals of the wireless devices are among the most power-intensive circuits of the wireless devices. Consequently, it is desirable that the amplifier circuits within wireless devices operate in both an efficient and a linear manner.
Power amplifiers are frequently utilized in wireless devices as the amplifier circuits for generating the output signals. While maximum efficiency is achieved by such power amplifiers when the power amplifiers are saturated, saturation of the power amplifiers also causes distortion of the output signals and generates interference outside the transmission bandwidth. Thus, some wireless devices include additional envelope detector circuits that are coupled to the power amplifiers in order to bias the power amplifiers toward saturation but within a range of operation in which the power amplifiers are both relatively efficient and linear in operation.
The envelope detector circuits monitor the envelopes or amplitudes of the input signals that are being amplified, and often provide control signals to vary the supply voltages applied to the power amplifiers based upon the envelopes. In particular, the control signals reduce the supply voltages when the envelopes of the input signals are smaller, and increase the supply voltages when the envelopes of the input signals are larger. By controlling the supply voltages that are applied to the power amplifiers, the envelope detector circuits keep the power amplifiers operating within the desired range allowing for efficient and linear operation.
Although the use of envelope detector circuits can guarantee efficient and linear operation of the power amplifiers under many circumstances, the use of envelope detector circuits results in nonlinear operation or even shutdown of the power amplifiers when the amplitudes of the envelopes that are being detected become small.
The present inventors have recognized that a predistortion circuit can be added to a conventional envelope detector circuit to avoid nonlinear operation or shutdown of a power amplifier. The predistortion circuit allows normal operation of the envelope detector circuit when the input signal to the envelope detector circuit has an envelope that is sufficiently great such that the resulting control signals produced by the envelope detector circuit do not cause the power amplifier to become nonlinear or to shutdown. However, the predistortion circuit causes the envelope detector circuit to produce a control signal that is at or above a minimum threshold when the input signal to the envelope detector circuit has an envelope that is sufficiently small such that, in the absence of the predistortion circuit, the control signals produced by the envelope detector circuit would cause the power amplifier to become nonlinear or shutdown.
In particular, the present invention relates to an envelope detector circuit that preferably includes a first semiconductor device, a first current drain and a second semiconductor device. The first semiconductor device has a first input port that receives a first input signal and a first output port that provides current to charge a capacitor in response to the first input signal. The first current drain is coupled to the first semiconductor device and the capacitor, and conducts current away from the capacitor. The second semiconductor device has a second input port that is set to a biasing voltage and a second output port that is coupled to the first output port of the first semiconductor device. A voltage level of the first output port is indicative of a level of an envelope of the first input signal when the voltage level remains above a threshold voltage equaling the biasing voltage minus a semiconductor voltage, and the voltage level otherwise does not fall below the threshold voltage.
The present invention further relates to, in an envelope detector circuit including preferably a first semiconductor device with a first input port and a first output port, a capacitor and a current drain, where the first output port provides current to charge the capacitor in response to a first input signal provided at the first input port, and the current drain conducts current away from the capacitor, the improvement comprising a predistortion circuit. The predistortion circuit includes a second semiconductor device having at least one port coupled to a biasing voltage and a second port coupled to the first output port of the first semiconductor device. The second semiconductor device provides a signal at the second port which prevents the first output port from falling below a threshold.
The present invention additionally relates to an envelope detector circuit comprising a means for producing an output signal that is indicative of a level of an envelope of an input signal, and a means for limiting the output signal so that the output signal does not fall below a threshold. The output signal is indicative of the level of the envelope of the input signal when the output signal is above the threshold.
The present invention further relates to a method of preventing at least one of nonlinear operation and shutdown of an RF amplifier due to a reduction in a level of an envelope of an input signal to the RF amplifier, where the RF amplifier is biased based at least in part upon the level of the envelope. The method comprises providing an envelope detector circuit, where preferably the envelope detector circuit includes a semiconductor device having an output port that is coupled to a capacitor and a current drain, and the output port is capable of providing an output signal indicative of the level of the envelope of the input signal. The method further comprises coupling a predistortion circuit to the output port of the semiconductor device, and receiving the input signal at the envelope detector circuit. The method additionally comprises maintaining the output signal above a minimum value when the input signal becomes sufficiently small that, in the absence of the predistortion circuit, the output signal would fall below the minimum value, and providing a biasing signal to the RF amplifier, where the biasing signal is functionally related to the output signal.
The present invention additionally relates to amplifier circuits that include envelope detector circuits.